1. Field of the invention
The present invention relates to a plastic lens, a method for manufacturing the lens, and an imaging device using the lens and, more particularly, an imaging system lens employed in a vehicle-mounted camera, a monitoring camera, an underwater camera, and the like.
2. Description of the related art
A glass lens has hitherto been used as a first surface lens to be fitted to the outermost surface of a vehicle-mounted camera. A common method employed in fixing a glass lens (a lens 10) is a so-called thermal caulking technique for thermally softening a portion of a lens holder and holding a lens tight in the holder. FIGS. 10(a) to 10(c) are drawings showing processes of thermal caulking. FIG. 10(a) shows a state achieved before thermal caulking; FIG. 10(b) shows a state achieved in the course of thermal caulking; and FIG. 10(c) shows a state achieved after thermal caulking.
As shown in FIG. 10(a), an annular thermal caulking portion 12 is provided in a lens holder 11 formed from a thermoplastic resin. The lens 10 is fitted into the thermal caulking portion 12, and a thermal caulking press member 20 is subsequently heated. The thermal caulking press member 20 is attached to the thermal caulking portion 12 from above and comes under predetermined pressure, whereupon, as shown in FIG. 10(c), an edge of the thermal caulking portion 12 covers an upper portion of the lens 10. Caulking is thus completed.
In the lens 10 having a flange 3 that is continual over an entire circumference, the thermal caulking portion 12 of the lens holder 11 can cover the circumference of the flange 3 of the lens 10 with no space therebetween. Accordingly, the thermal caulking portion 12 can evenly bear load, and given lens anchorage strength or more can be readily fulfilled.
In the meantime, another proposed lens 10 has the flange 3 that is partially cut rather than being formed continuously along the entire circumference of the lens 10. As shown in FIGS. 11(a) and 11(b), a D-cut shape resulting from partial chipping of the flange 3 is also proposed for the lens 10 that does not have the continuous flange 3. However, the configuration includes clearance between the thermal caulking portion 12 of the lens holder 11 and the flange 3 of the lens 10. Hence, the circumference of the lens cannot evenly bear load, so that sufficient lens anchorage strength cannot be obtained.
When the first surface lens of the vehicle-mounted camera is replaced with a plastic lens as a substitute for a glass lens for reasons mentioned above, it is preferable that a circumference of the plastic lens be provided with a circumferential flange 3 which is of a given thickness or more.
Incidentally, in relation to a plastic lens formed by means of injection molding, injection compression molding, or the like, when molten plastic is injected into a cavity (a molding space) from a side, the molten plastic is left at a gate due to presence of an inlet (the gate), whereby a projecting gate is formed integrally on the circumferential flange.
As described in connection with; for instance, JP-A-11-202106, a gate 4 of the plastic lens is generally provided on the circumference of the flange 3 as shown in FIG. 12 or on a D-cut surface 6 of the flange 3 as shown in FIG. 16.
Since the gate 4 is originally unwanted, work for separating the gate 4 from the plastic lens; namely, a so-called gate cut process, is necessary. Detailed descriptions are provided to cutting of the gate by reference to FIGS. 12(a), (b), and (c).
The gate 4 provided on the circumference of the flange 3 has hitherto been cut at any point on (a root of) the gate 4 as depicted as a cut plane 5 in FIGS. 12(a), (b), and (c). Alternatively, the gate 4 is cut, while including a portion of the flange 3, along a cut plane 6 shown in FIGS. 12(a), (b), and (c). FIGS. 13(a), (b), and (c) are drawings showing an idealistic cut state of a cut made along the cut plane 6. In FIGS. 12 and 13, (a) shows front views, (b) shows a right side elevation of (a), and (c) shows rear views.